Are the actions of a husband's mother towards her 26-year-old son—such as hugging him, kissing him on the mouth, and feeding him—natural and permissible (halal), and is the wife's objection to this justified?

An mother embracing and kissing her adult son on his mouth out of affection is unacceptable. Some scholars have explicitly prohibited kissing on the mouth for anyone other than one's wife. Exceptions are made for kissing on the forehead or head upon returning from a journey or similar circumstances. Therefore, the husband should be gently advised to consult the words of the scholars regarding interaction with unmarriageable kin (mahrams), while being careful to honor and treat his mother with kindness.